The size is dependent on your camping style and goal for the solar. If all you want to do is keep your battery topped off 40w system may fill the bill. If boon-docking and wanting to recharge batteries then a much larger system is needed. Theft (with anything unlocked outside) is more of a location problem in my estimation, camping in Idaho not so much in other states may be different

I am also going portable. Plan on using a “toy lock” type cable and a chain thru the legs.
The chain is more for the weight to keep it from blowing over than anything else. I have chosen the Renology 100w with the water proof Voyager controller. It is the only waterproof unit other than Zamp and the kit is 1/2 the price. I also plan on 20’ of cable so I can get full sun.

The controller should just be mounted as close to the batteries as possible so the fact that it is for a portable system or not, shouldn't make a difference if it is waterproof.

By mounting it close to the batteries, you also give yourself an option to later putting a panel in parallel to your portable one on your camper roof.

The subject portable panel(s) is a suitcase and has an integrated controller. Waterproof makes a huge difference.
The object is “plug and play” utilizing the Zamp prewired plug these units come with.
If you want to use a plain panel then you would need to install
a controller and your suggestion would be a good idea.

Have had roof mounted solar 200W. Going to purchase a 3050S and am considering portable panel. Would like some input as to size and has there been any issues with theft?

I started with the Renogy 100 watt Suitcase with myClass C Minnie Winnie. When we bought our Forester, I continued using the Suitcase. It only puts out about 5.3 amps at best. I was worried about it growing legs and never left it unattended. Any lock or cable can easily be cut by bolt cutters. So it was limited as to how much charging it can do. I have since added 2 100 watt Renogy panels and found it only got my batteries back to 80-90%. I have since added another 100 watt panel and have about 16 amp max output on the roof. I added a connector to the controller just outside my step so I can connect my Renogy 100 watt suitcase directly to the controller bringing my max to almost 22 amps. I did have to rewire the suitcase to bypass its 10 amp controller. Nice to have the suitcase incase we are camped in shade and or need more charging output. We use about 50-70 amps per day, so the 400 watt 22 amps does the job recharging.